HEAT TREATMENT. 287
has been heated thoroughly can only be discovered by practice and a knowledge of the heating capacity of the furnace. As good a way perhaps as any is to note the time of heating to a certain indicated temperature, then cool under conditions which may be duplicated and note time of cooling; then heat to this temperature again, soak for some time and cool under previous conditions, and if the cooling takes longer the piece is heated more nearly uniformly. After a few trials in this way the necessary time may be estimated with sufficient accuracy. It may seem that this is an unnecessary refinement, but up to the present time, except in a limited number of grades of steel and at a,few works, proper attention has not been given to the annealing of steel.
SEC. XVi.—Definition of the term "critical point"—It a piece of steel containing over 0.50 per cent, of carbon be allowed to cool slowly from a high temperature, certain peculiar phenomena will be noticed. The cooling at first proceeds at a uniformly retarded rate, but when a temperature of about 700° C. is reached there is an interruption of this regularity. In some cases the rate of cooling may become very slow, in other cases the bar may not decrease in temperature at all, while in still other cases the bar may actually grow hotter for a moment in spite of the fact that it is free to radiate heat in every direction and that it has been cooling regularly down to that particular temperature. Moreover, it will be found that when this "critical point" is passed, the bar cools as before until it reaches the temperature of the atmosphere. It is, of course, a matter of common knowledge that a bar will cool in less time from 1000° C. to 900° C. than it will from 200° C. to 100° C. and the term "uniformly retarded," as above used, is intended to cover this fact.
It is quite clear that there must be some change taking place within the metal itself giving rise to heat, and any point at which such an action takes place in any steel is called a "critical point" and in metallography such a point is denoted by the letter A, the particular one just described in which there is a retardation in the cooling of a piece of steel being denoted by the term Ar. In heating a piece of steel through this range of temperature, we naturally encounter an exactly opposite phenomenon, there being an absorption of heat by internal molecular reaction, with a consequent retardation in the rise of temperature, and this point is called Ac. It has been shown by Prof. Howe that Ac is some 30° C. higher